Such a system is known from U.S. Pat. No. 4,238,778. The known system can make known the presence of an approaching emergency service vehicle (for instance an ambulance), to adjust speed depending on the warning signals of the emergency service vehicle being heard and seen.
In the known system, the emergency service vehicle is provided with a separate transmitter, which transmits a radio-frequency warning signal. The warning signal has a predetermined frequency, for instance in the VHF or UHF range. Another motor vehicle is provided with a receiver, arranged to receive the radio-frequency warning signal transmitted by the emergency service vehicle. The receiver tuned to the predetermined frequency delivers an acoustic signal to warn a driver of the motor vehicle, upon reception of the warning signal.
According to U.S. Pat. No. 4,238,778, a communication possibility is preferably provided which allows a driver of the emergency service vehicle to give commands to the driver of a motor vehicle to be warned, via the transmitter. An advantage is that the driver of a motor vehicle can be alerted to the approach of an emergency service vehicle in a timely manner and can take appropriate measures, for instance by timely moving to the side.